The present invention concerns a method and an arrangement for radiocommunications. The invention is designed in particular to be used for radio receivers, for example radar systems, for reducing distortion in these types of receiver.
In connection with radiocommunications, a type of radio receiver often used is that erected around a an antenna array for receiving and handling radio signals. This type of antenna is normally used, for example, with radar systems. Based on established technology, such an antenna array consists of a number of antenna elements which are connected to a corresponding number of amplifiers. The received and amplified signals are then fed through electrical circuits, normally in the form of mixers, filters and A/D converters. In this way the antenna can be said to be formed of a number of receiver channels, each of which is connected to one or more antenna elements with their associated electrical circuits, whereby the output signals from the respective channels are suitably processed and handled in digital signal-processing circuits.
One problem that can arise in connection with previously known radio receivers which are based on antenna arrays concerns the fact that the various receiver channels have non-linearities and other imperfections, for example, timing errors, gain errors and phase errors, which in different ways give rise to undesirable distortion. For instance, the distortion can lead to intermodulation products between adjacent frequencies, insufficient suppression of mirror components and the generation of undesirable harmonies in the received radio signals. These imperfections will of course constitute problems in radiocommunications systems.
Based on established technology, the aforementioned problem of distortion in the form of non-linearities can be solved to a certain extent by filtering the received signals, e.g. by using bandpass filters which are placed downstream the respective amplifier in the respective receiver channel. In this way, for example, the intermodulation products which lie outside the transmitted signal band in question can be attenuated. Such filters are however limited in that they do not provide continuous suppression of undesirable signals. The result of this is that signals lying outside the frequency band in question will to a certain extent leak through and have a negative effect on the desirable received signals. In an A/D converter, for example, these undesirable signal components, when mixed down to baseband, are diverted into the frequency range which is used for subsequent handling of the received signals.
Another solution to the abovementioned problem is to use an active antenna which includes amplifiers with a very high degree of linearity. A disadvantage of this solution is, however, that it demands a relatively high power consumption from the amplifiers in question, which in turn results in amplifiers with relatively high running costs. However, this will not in itself solve the problem of mirror frequencies which can give rise to distortion after mixing down to baseband.
The problems connected with distortion in antenna arrays can be illustrated by the following. In the case of a receiver system which consists of quantity M antenna elements and receiver channels, the following applies. During spatial beam-formation, which can be either analogue or digital, the signals from the respective channels are added, whereby the signals can be weighted according to the following equation:       z    ⁢          xe2x80x83        ⁢          (      t      )        =            ∑              m        =        1            M        ⁢          xe2x80x83        ⁢                            W          m                ·                  X          m                    ⁢              xe2x80x83            ⁢              (        t        )            
where Xm (t) is made up of the signal in the mth channel at time t, which includes the desired received signal as well as distortion. The weighted values Wm are complex and are chosen based on the properties which are required in the resultant signal z(t), for example formation of sidelobes and the antenna""s desired sensitivity in a certain direction. Even if the distortion in itself is small, it can, after summation according to the equation above, reach energy levels which exceed the threshold which is set in a radar system when a target is detected. In this way, for example, a false target can be detected by a radar system using this technology, which will of course constitute a problem.
In patent document EP 0545742, an arrangement is shown for reducing overtones generated by non-linearities in the amplifiers of an active antenna. In this arrangement, antenna elements with their associated amplifiers are split into at least two groups. A common phase shift xcfx86 is added to each of the amplifier groups. In one of the two groups, the phase shift is added to the signal before the respective amplifier, and in the second group the phase shift is added after the respective amplifier. In this way, a reduction in the level of the overtones is achieved.
Patent document SE 468029 describes a method for reducing the effect of distortion in a signal-processing system for analogue-digital conversion of samples of an electrical signal. According to this method two quadrature signals are formed before the conversion, whereby these signals are phase-modulated before conversion and phase-demodulated after conversion. In this way the fundamental frequency in the signal will be correctly demodulated while any harmonies and intermodulation products will have residual phase modulation after the phase demodulation.
In the process, the undesirable signal components can be suppressed by means of filtering.
The object of the present invention is to offer an improved method for radiocommunications, in particular for reducing the distortion which occurs in an antenna array with several receiver channels. This object is achieved by means of a method and an arrangement.
The invention is designed for reducing distortion in a radiocommunications system which includes reception of a radio signal at a carrier frequency with an antenna array which includes at least two receiver channels with an antenna element and components for amplifying and processing respective signals. According to the invention, the received signal in the respective receiver channel is also amplified. According to the invention there will additionally occur a first phase shift of a predetermined value also takes place directly after reception of the said signal in the respective channel, and a second phase shift with a value which corresponds to the said first phase, shift, but in the opposite direction.
According to the invention, the real carrier-frequency signals in the respective receiver channel are given a phase shift xcex2m, where m=1, . . . , M, which is referred to below as xe2x80x9cspatial phase codingxe2x80x9d. After the formation of complex signals, the phase is shifted back in the respective receiver channel, chiefly by means of complex multiplication by exp(xe2x88x92jxcex2m).
Because of the method according to the invention, only those frequency components which are originally in the desired frequency band around xcfx89o will be completely restored. Other frequency components will have varying xe2x80x9cresidualxe2x80x9d valves (i.e. a residual phase shift) remaining from the phase shift added initially. Because of this residue it will be possible to reduce the distortion when adding the channels in the receiver in question.
The choice of the phase shift xcex2m can be varied depending on the requirements, and will be clarified when reference is made to the embodiments of the invention below.